US government UFO studies:

At various stages of UFO history, the Air Force high command considered UFOs to be possibly extraterrestrial spaceships... and at the other extreme an annoying public relations problem. Even Air Force officers at the Air University, Air Command and Staff College, wrote reports puzzling over the Air Force position and raising serious issues about the significance of UFO data. The huge mass of serious reports and the impressive collection of declassified documents, as well as numerous military witnesses who went public leaves little doubt: there are UFOs, and they are not ours.

Project "Grudge":

Project Grudge replaced Project Sign in April 1949. In December 1949, a magazine article on UFOs written by the famous aviation writer, Donald Keyhoe, based on his private investigations and military contacts, elicited enormous media attention. In it, Keyhoe insisted that UFOs were alien spacecraft and that the U.S. Government was keeping this knowledge secret. In response to the furor that Keyhoe's article caused, and to demonstrate that there was nothing to get excited about, the Air Force reduced Project Grudge to a routine intelligence effort. However, in October 1951, Project Grudge was returned to its original status as a special project. This investigation ended in March 1952. The final report suggested that most sightings had been explained. However, a large percentage of the reports were left either unexplained or only conditionally explained.

Sample documents:

Memo from SAC to FBI, one of the many document about the numerous sightings of the "green fireballs" in New Mexico, indicating that these phenomenon are real, are classified secret, are considered undientified flying objects, and a threat to the vital installation they fly over.

May 25, 1950

A report: in 1949, two confidential conferences between scientists, intelligence personal, were held at Los Alamos, New Mexico, to evaluate what the "green fireballs" were. Conclusion: they were not explainable in terms of common phenomenon. Theory: interstellar probes from another planet.